1. Field of the Invention
The present invention relates to a cavity structure for use in a nitrogen, especially in an explosive detection system. Specifically, the present invention relates to a composite cavity structure which in combination with a source of neutrons produces a cloud of thermal neutrons within the cavity to provide for a more efficient detection of the nitrogen contained in an object (such as an explosive) within the cavity.
2. Description of the Prior Art
A great need exists for the scanning of luggage, baggage and other parcels for the detection of any explosive material contained or concealed within their confines. For example, a large number such as close to two million (2,000,000) pieces of luggage are checked and/or carried onto aircraft daily by close to seven hundred and fifty thousand (750,000) passengers within six hundred (600) airports extending across the country. There is a possibility, albeit remote, that any one piece of luggage or parcel may contain explosive material.
It is, therefore, desirable to protect the public by providing detection systems to scan luggage and parcels to detect the presence of any explosive material. Co-pending application Ser. No. 053,950 filed by Tsani Gozani and Patrick M. Shea on May 26, 1987 (now abandoned in favor of continuation Ser. No. 321,511 and filed Mar. 9, 1989) for "Explosive Detection System" and assigned to the same assignee of the present application, is directed to an overall detection system to provide for the checking of luggage or parcels for explosives with a high probability of detection and a low probability of false alarms. It is appreciated that any system should have a high probability of detection in order to be effective.
It is also appreciated that any detection system, because of the large number of passengers, is bound to occasionally give a false alarm. The probability of these false alarms must be minimized in order to provide for an effective explosive detection system. This is true, because when an alarm occurs it is not known at that time whether is it proper or false. This means that each time an alarm occurs a passenger may be detained for further investigation. If false alarms are significantly high, the nuisance level and the delays can be unacceptable to passengers. It is, therefore, important that any explosive detection system should have a very high probability of detection and yet at the same time have a very low probability of false alarms. These conflicting criteria have hampered efforts in the past to build a reliable and usable system.
The prior art systems have not had the desired characteristics of having a high probability of detection with a low probability of false alarms. As an example, one such prior art system is shown in U.S. Pat. No. 3,832,545. This patent provides for a system for the detection of nitrogen which is generally present in the explosive materials to be detected. The object under observation is positioned within a cavity structure and is bombarded by thermal neutrons. The thermal neutrons react with any nitrogen contained in the object to provide for the emission of gamma rays at an energy level characteristic of the presence of nitrogen. The emitted gamma rays are then detected by an array of gamma ray detectors.
The prior art U.S. Pat. No. 3,832,545 specifically provides for the use of liquid and plastic type organic scintillator detectors. These detectors are provided in an array to produce a two dimensional profile of the nitrogen content within the object being inspected.
Co-pending application Ser. No. 053,950 filed by Tsahi Gozani and Patrick M. Shea on May 26, 1987 (now abandoned in favor of continuation Ser. No. 321,511 filed Mar. 9, 1989) for "Explosive Detection System" and assigned of record to the assignee of record of this application discloses and claims a system which is more effective than the system of U.S. Pat. No. 3,832,545. The system of co-pending application Ser. No. 321,511, provides for the use of inorganic scintillators as detectors. These inorganic detectors are formed as a C-ring so as to provide for a detection of a slice or plane of the object under inspection. The object is moved continuously through the C-ring of detectors so as to provide for a plurality of slices or parallel successive planes. The parallel successive planes may then be used to build a three dimensional profile of the concentration of the nitrogen contained within the object under inspection.
The composite cavity structure of the present invention may be used with either the prior art organic scintillators in an array or the C-ring array of inorganic scintillators shown in the co-pending application. However, the present invention is described with reference to the use of the C-ring array of inorganic scintillators of the co-pending application.
The detection of the explosive should be independent of the specific configuration and must be non-intrusive in order to protect privacy. The detection equipment, of course, must be non-hazardous to the contents of the checked items and to the operating personnel and environment. Other more general criteria are that the system must be reliable, easily maintained and operable by relatively unskilled personnel and that the cost must be low enough so as to be non-burdensome to airlines and airports. Finally, the size of the system must be relatively small so that the system may be useful in a wide variety of environments.
In order to develop a proper explosive detection system, an understanding of the properties of the various explosives are relevant to the specific techniques to be used. Although there are a large number of explosive types, a general classification into six major groups with minor variations, has been proposed. The proposed classification scheme includes the following types of explosives: (1) nitroglycerine based dynamites, (2) ammonium nitrate based dynamites, (3) military explosives, (4) homemade explosives, (5) low order powders, and (6) special purpose explosives.
In general, all of these explosives contain a relatively high amount of nitrogen concentration ranging from nine to thirty five percent by weight and with a normal concentration range between fifteen to thirty five percent with twenty percent as a typical value. The nominal density of these explosives is typically 1.6 g/cm.sup.3 and with a range from 1.25 to 2 g/cm.sup.3 or more. These physical properties demonstrate that the most unique signature of explosives is the high concentration and density of the nitrogen content.
In can be seen, therefore, that a nuclear detection technique can provide for the detection of the nitrogen content to reliably indicate the presence of a large nitrogen content. However, the universal occurence of nitrogen in non-explosive materials limits the level of detection sensitivity and merely detecting the presence or absence of nitrogen alone is not sufficient. Therefore, additional information is required beyond simply sensing the presence of the nitrogen. The present invention provides for a composite cavity structure which enhances the production of this additional information using specific structures and materials for the cavity.